Memory erase and memory read-out in an EL display panel controlled by an electron beam

ABSTRACT

An EL display panel comprising an electroluminescent element made of, for example, a ZnS:Mn layer sandwiched between a pair of dielectric layers exhibits hysteresis properties within light intensity versus applied voltage characteristics. A front electrode is formed on one of the dielectric layers, and a rear electrode is formed on the other dielectric layer in order to apply a sustaining voltage signal across the electroluminescent element for maintaining the memoried display information. An electron beam is applied to a desired position on the EL display panel through the rear electrode at a time when the sustaining voltage signal bears the zero level in order to erase the memoried information. The memoried display information is electrically read out by detecting a polarization relaxation current which flows through a memoried display position when an electron beam is applied thereto.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a drive system of an EL display paneland, more particularly, to a memory erase drive and memory read-outdrive in an EL display panel which shows memory characteristics.

A thin-film electroluminescent element of a three-layer construction iswell known in the art, which comprises a semiconductorelectroluminescent thin film made of, for example, a ZnS layer dopedwith Mn (ZnS:Mn) and a ZnSe layer doped with Mn(ZnSe:Mn) sandwichedbetween a pair of dielectric thin films made of Y₂ O₃, Si₃ N₄ or TiO₂.The above-mentioned thin-film electroluminescent element exhibitselectroluminescence of high brightness upon receiving A.C. voltagesignal of several kilohertz. And, the above-mentioned thin-filmelectroluminescent element shows the long life operation.

By properly controlling the amount of Mn doped within theelectroluminescent layer and the fabrication conditions, the aboveconstructed thin-film EL element exhibits the hysteresis propertieswithin light intensity versus applied voltage characteristics asdisclosed in Y. KANATANI et al, U.S. Pat. No. 3,967,112, "PHOTO-IMAGEMEMORY PANEL AND ACTIVATING METHOD THEREOF" on June 29, 1976.

Generally, when light energy, an electric field or heat energy isapplied to the thin-film EL element having the hysteresischaracteristics under a condition where the applied voltage isincreased, the thin-film EL element is excited to exhibit light emissioncorresponding to the applied energy. The thus obtained light emission isheld or memoried even after the application of the light energy, theelectric field or the heat energy is terminated. By effectivelyutilizing the above-mentioned memory phenomenon, the thin-film ELelement can be applied to various technical fields.

Accordingly, an object of the present invention is to provide a noveldrive system for a thin-film EL element having hysteresischaracteristics.

Another object of the present invention is to provide a drive system forerasing memoried information in a thin-film EL element having hysteresischaracteristics.

Still another object of the present invention is to provide a drivesystem for reading-out memoried information in a thin-film EL elementhaving hysteresis characteristics.

Yet another object of the present invention is to combine a sustainingvoltage signal with an electron beam erase signal in a thin-film ELdisplay panel which shows hysteresis characteristics.

A further object of the present invention is to combine a sustainingvoltage signal with an electron beam read-out signal in a thin-film ELdisplay panel which shows hysteresis characteristics.

Other objects and further scope of applicability of the presentinvention will become apparent from the detailed description givenhereinafter. It should be understood, however, that the detaileddescription and specific examples, while indicating preferredembodiments of the invention, are given by way of illustration only,since various changes and modifications within the spirit and scope ofthe invention will become apparent to those skilled in the art from thisdetailed description.

To achieve the above objects, pursuant to an embodiment of the presentinvention, a thin-film EL display panel is provided which comprises athin-film EL layer made of, for example, a ZnS:Mn thin film sandwichedbetween a pair of dielectric layers made of Y₂ O₃, Si₃ N₄ or TiO₂. Afront transparent electrode made of SnO₂ or In₂ O₃ is formed on one ofthe dielectric layers, and a rear metal electrode made of, for example,aluminum is formed on the other dielectric layer. The thus formedthin-film EL element is supported by a glass substrate in such a mannerthat the front transparent electrode confronts the glass substrate.

The above constructed thin-film EL display panel is disposed at adisplay surface of a cathode-ray tube in such a manner that the glasssubstrate is exposed to the outside. An electron beam generator isdisposed at an end of the cathode-ray tube for applying an electron beamto the thin-film EL display panel through the rear metal electrode.

An alternating sustaining voltage signal is applied to the thin-film ELelement through the use of the front and rear electrodes in order tomaintain the information displayed on the thin-film EL display panel. Anelectron beam is applied from the electron beam generator to a desiredposition on the thin-film EL display panel at a time when the sustainingvoltage signal bears the zero level, thereby erasing the memoriedinformation.

The memoried display information is electrically read out by detecting apolarization relaxation current which flows through a memoried displayposition when an electron beam is applied thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention and wherein:

FIG. 1 is a schematic perspective view of a basic construction of athin-film EL element employed in an embodiment of the present invention;

FIG. 2 is a graph showing hysteresis properties included withinelectroluminescent brightness versus applied voltage characteristics ofthe thin-film EL element of FIG. 1;

FIGS. 3(A) through 3(E) are time charts for explaining basic operationof an embodiment of a drive system of the present invention;

FIG. 4 is a block diagram of an essential part of an embodiment of adrive system of the present invention;

FIG. 5 is a block diagram of an embodiment of a drive system of thepresent invention;

FIG. 6 is a schematic diagram of another embodiment of a drive system ofthe present invention;

FIG. 7 is a circuit diagram of an equivalent circuit of a thin-film ELelement employed in the present invention;

FIG. 8 is a circuit diagram of an embodiment of a read-out signalgenerator employed in the drive system of FIG. 6;

FIGS. 9(A) through 9(F) are time charts for explaining operation of thedrive system of FIG. 6;

FIG. 10 is a circuit diagram of an embodiment of a polarization currentdetection circuit employed in the drive system of FIG. 6;

FIGS. 11(A) through 11(F) are time charts for explaining read-outoperation; and

FIG. 12 is a block diagram of the drive system of FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a basic construction of a thin-film EL element employed inan embodiment of the present invention.

A transparent electrode 2 made of, for example, SnO₂ or In₂ O₃ is formedon a glass substrate 1. A bottm insulation thin-film layer 3 made of,for example, Y₂ O₃ or Si₃ N₄ is formed on the transparent electrode 2,and a thin-film ZnS electroluminescence layer 4 doped with Mn is formedthereon. Thereafter, an upper insulation thin film layer 5 made of asame material as the layer 3 is formed on the thin-film ZnSelectroluminenscence layer 4 to sandwich the thin-film ZnSelectroluminescence layer 4 between the bottom and upper insulation thinfilm layers 3 and 5, and a rear metal electrode 6 made of, for example,aluminum is formed thereon. The bottom insulation thin film layer 3, thethin-film ZnS electroluminescence layer 4 and the upper insulation thinfilm layer 5 are sequentially formed through the use of, for example, anevaporation method or a sputtering method. The transparent electrode 2and the rear metal electrode 6 are formed so as to cover the entiresurface of the element, and are connected to an alternating voltagesource 7 through lead wires.

FIG. 2 is a graph showing hysteresis properties included withinelectroluminescent brightness versus applied voltage characteristics ofa thin-film EL element employed in an embodiment of the presentinvention. The electroluminescent brightness [B] is shown along theordinate axis, and the peak value [V] of the applied alternating voltagepulse signal is shown along the abscissa axis.

It will be clear from FIG. 2 that the hysteresis loop is observed in thevoltage increasing curve I and the voltage decreasing curve II.

Preferably, a sustaining voltage V_(s) is selected at a level where thedifference is sufficiently large between the brightness B_(e) in thevoltage increasing curve and the brightness B_(w) in the brightnessdecreasing curve. FIG. 3(A) shows the waveform of the sustaining voltagesignal. The brightness B_(e) will be referred to as an erase brightnessB_(e), and the brightness B_(w) will be referred to as a write-inbrightness B_(w), respectively, hereinafter.

When an alternating sustaining pulse train P_(s) (peak value V_(s)) asshown in FIG. 3(A) is applied to the thin-film EL element, the thin-filmEL element exhibits the electroluminescence of the erase brightnessB_(e) at a point S of FIG. 2. The erase brightness B_(e) is maintainedby the alternating sustaining pulse train P_(s).

When an amplitude of the alternating sustaining pulse train P_(s) ismomentarily increased, or when the light energy or the heat energy ismomentarily applied to the thin-film EL element under the conditionwhere the alternating sustaining voltage pulse is applied thereto, thebrightness is momentarily increased to the level B'_(w) corresponding tothe point P of FIG. 2 and, thereafter, the brightness is held stationaryat the write-in brightness B_(w) corresponding to the point Q on thevoltage decreasing curve II by the following alternating sustainingpulse (luminescence memory condition).

When the light energy or the heat energy is applied to the thin-film ELelement at a time when the pulse train P_(s) of the alternatingsustaining voltage V_(s) is applied thereto, or when a voltage higherthan the alternating sustaining voltage V_(s) is applied to thethin-film EL element, electrons captured in the electron trap levelwithin the thin-film ZnS layer 4 of the thin-film EL element are excitedto conduction by the number corresponding to the applied energy. Thethus generated conduction electrons travel through the thin-film ZnSlayer 4 and function to excite the Mn luminescent center formed in thethin-film ZnS layer 4. Therefore, the electroluminescent brightness ofthe thin-film EL element is increased.

When the amplitude of the alternating sustaining pulse applied to thethin-film EL element held in the luminescence memory condition ismomentarily reduced to an erase voltage V_(e), or when the light energyor the heat energy is applied to the thin-film EL element held in theluminescence memory condition at a time when the alternating sustainingpulse bears the zero level, the brightness of the thin-film EL elementis momentarily reduced to the erase brightness B_(e) corresponding tothe point R of FIG. 2. Thereafter, the erase brightness B_(e) ismaintained at the point S of FIG. 2 by the following alternatingsustaining pulse train P_(s) (erase memory condition).

Moreover, the brightness of intermediate tones can be obtained betweenthe write-in brightness B_(w) and the erase brightness B_(e) by theapplication of the sustaining voltage V_(s) when the energy level of thevoltage, light or heat applied to the thin-film EL element at thewrite-in operation or the erase operation is properly controlled.

The sustaining operation of the write-in brightness B_(w) and the erasebrightness B_(e) in the thin-film EL element by the sustaining voltageV_(s) are considered as follows.

When the light energy or the heat energy is applied to the thin-film ELelement at a time when the sustaining voltage V_(s) is applied thereto,or when the voltage higher than the sustaining voltage is applied to thethin-film EL element, the conduction electrons are swept toward theboundary area formed between the thin-film ZnS layer 4 and the thin-filminsulation layer 3 or formed between the thin-film ZnS layer 4 and thethin-film insulation layer 5. That is, the thin-film EL element ispolarized.

The thus swept conduction electrons are captured at the boundary levelof the boundary area formed between the thin-film ZnS layer 4 and thethin-film insulation layer 3 or 5 even after the application of thelight energy, the heat energy or the high voltage is removed. The thuscaptured conduction electrons are escaped from the boundary level to theconduction band by application of the following sustaining pulse, andtravel through the thin-film ZnS layer 4 toward the opposing boundaryarea. Most of the conduction electrons are swept to the opposingboundary area without being re-trapped at the electron trap level formedin the thin-film ZnS layer 4, because the conduction electrons travel ata high speed due to the electric field established by the sustainingpulse. Therefore, the thin-film EL element maintains the write-inbrightness B_(w), or maintains the luminescence memory condition. Inaddition, the polarization generated in the thin-film EL element is alsosustained.

When the light energy or the heat energy is applied to the thin-film ELelement held in the luminescence memory condition (or polarizedcondition) at a time when the sustaining voltage pulse takes the zerolevel, or when the erase voltage V_(e) is applied to the thin-film ELelement held in the luminescence memory condition (or polarizedcondition), the conduction electrons swept at the boundary area formedbetween the thin-film ZnS layer 4 and the thin-film insulation layer 3or 5 are released and, therefore, the polarization is relaxed.Accordingly, the sweep velocity of the conduction electrons travelingnear the electron trap level is low when the following sustaining pulseis applied to the element. Most of the conduction electrons arere-trapped at the electron trap level formed in the thin-film ZnS layer4 without reaching the opposing boundary area while they travel throughthe thin-film ZnS layer 4. The polarization is not formed again.Consequently, the thin-film EL element is held stationary in the erasememory condition to exhibits the erase brightness B_(e).

The sweeping ratio of the conduction electrons can be controlled byvarying the energy level applied at the erase operation. That is, thebrightness of the thin-film EL element can be controlled by varying theenergy level applied to the element.

In case where the electrodes are uniformly formed on the element tocover the entire surface of the element, clean display and memory can beobtained as compared with the element wherein the electrodes are formedin a matrix fashion. Moreover, the fabrication of the electrodes becomeseasy. However, it was impossible, in the prior art, to electrically readout the memoried information by selecting a desired position.

The present invention provides a system wherein an electron beam isapplied to a desired position on the thin-film EL element in order toerase the memoried information. In addition, the present inventionprovides a system wherein an electron beam is applied to a desiredposition on the thin-film EL element in order to read-out the memoriedinformation by detecting a polarization relaxation current flowingthrough the thin-film EL element.

FIG. 4 shows an essential part of an embodiment of a drive system of thepresent invention. FIGS. 3(A) through 3(E) show various signalsoccurrring within the system of FIG. 4. FIG. 3(A) shows a waveform of avoltage pulse train P_(s) applied from the alternating sustainingvoltage source 7 to the thin-film EL element; FIG. 3(B) shows anelectron beam exposure applied to the thin-film EL element; FIG. 3(C)shows a light emission waveform of the thin-film EL element; FIG. 3(D)shows polarization amount generated in the thin-film EL element; andFIG. 3(E) shows a current waveform flowing from a thin-film EL element 8to a load impedance 9.

When the thin-film EL element is placed in the erase memory condition(period I), only a transient current flows through the thin-film ELelement 8 in synchronization with the application of the voltage pulsetrain P_(s). When an electron beam is applied to the thin-film ELelement 8 in synchronization with the application of the sustainingvoltage pulse train P_(s), the thin-film EL element 8 exhibits the highbrightness and the polarization is generated in the element (period II,the luminescence memory condition). At this moment, the polarizationcurrent flows through the load impedance 9 in addition to the transientcurrent. The luminescence memory condition is sustained by thesustaining voltage V_(s) of the sustaining voltage pulse train P_(s)shown in FIG. 3(A).

When an electron beam is applied to the thin-film EL element 8 held inthe luminescence memory condition at a time when the alternatingsustaining voltage pulse train P_(s) takes the zero level, the photorelaxation phenomenon is developed at a position to which the electronbeam is applied. The erase operation is conducted as shown in FIG. 3(C).That is, the polarization charges stored in the boundary area of thethin-film EL element are relaxed by conduction electrons formed by theelectron beam application.

In this way, a desired portion of the displayed pattern is erased. Anegative display pattern can be easily obtained by applying the electronbeam in a desired pattern after the entire display surface of theelement is palced in the written condition.

In the above-mentioned period III, the polarization relaxation currentflows through the load impedance 9 in synchronization with theapplication of the electron beam.

Even when the electron beam is applied to the thin-film EL element 8held in the erase memory condition at a time when the alternatingsustaining voltage signal takes the zero level (period IV), thepolarization relaxation current does not flow through the load impedance9. The intensity of the electron beam can not be too strong, because thestrong electron beam applied to the thin-film EL element 8 will placethe thin-film EL element 8 in the luminescence memory condition evenwhen the electron beam is applied to the element at a time when thesustaining voltage signal takes the zero level.

The polarization relaxation current value will be proportional to thepolarization amount when the thin-film EL element 8 exhibits the lightemission of the intermediate tone between the write-in brightness B_(w)and the erase brightness B_(e).

Accordingly, the information stored at the position on the thin-film ELelement to which the electron beam is applied at a time when the appliedvoltage takes the zero level is read-out by detecting the polarizationrelaxation current flowing through the load impedance 9.

Referring now to FIG. 4, the alternating sustaining voltage source 7 andthe load impedance 9 such as a resistor R are connected to the thin-filmEL element 8 in a series fashion. A gate circuit 11, which selectivelytransfers the polarization relaxation current signal, is connected tothe thin-film EL element via an amplifier 10 (although the amplifier 10is not necessarily required). The gate circuit 11 is controlled by atiming pulse 12, which controls the application timing of thealternating sustaining voltage pulse, and by another timing pulse 13,which controls the application timing of the electron beam. Thepolarization relaxation current signal 14 developed from the gatecircuit 11 is applied to a recorder or an indicator (not shown).

The selective transfer of the polarization relaxation current signal 14,which indicates the information stored in the thin-film EL element 8, iscontrolled in the following manner. The timing pulse 12 is developed ata time when the alternating sustaining pulse signal bears the zerolevel, and the timing pulse 13 is developed at a time when the electronbeam is generated. The gate circuit 11 is opened only when the timingpulses 12 and 13 are simultaneously applied thereto. The polarizationrelaxation current signal 14 incidates the information stored at theposition in the thin-film EL element 8 to which the electron beam isapplied.

FIG. 5 shows an embodiment of a drive system of the present invention,which controls the above-mentioned erase operation and read-outoperation.

The thin-film EL element 8 having the same construction as shown in FIG.1 is disposed at a display surface of a cathode-ray tube 15. That is,the glass substrate 1 defines the front surface of the cathode-ray tube15. A focus control electro-magnetic coil 16 and an X-Y deflection coil17 are disposed as is well known in the art. The focus controlelectro-magnetic coil 16 is connected to receive a control signalderived from an electron beam focus control signal generator 18, and theX-Y deflection coil 17 is connected to receive control signals derivedfrom an X-direction deflection amplifer 19 and a Y-direction deflectionamplifier 20. The amplifiers 19 and 20 are connected to receive signalsderived from a scanning signal generator 22, which is connected toreceive a video signal derived from a modulator 21.

The transparent electrode 2 and the rear metal electrode 6 of thethin-film EL element 8 are connected to receive the sustaining pulsevoltage and the erase pulse voltage derived from the alternatingsustaining voltage source 7 and an erase signal generator 23,respectively. The erase signal generator 23 is provided for conductingvoltage controlled erase operation, and is not necessarily required. Thealternating sustaining voltage source 7 and the erase signal generator23 are connected to receive a synchronization signal derived from thescanning signal generator 22. An electron beam generator 24 is disposedat the end of the cathode-ray tube 15. The electron beam generator 24 isconnected to receive a brightness control signal derived from thescanning signal generator 22.

An inverter circuit 25 is connected to the scanning signal generator 22.The inverter circuit 25 develops the timing pulse 12 to be applied tothe gate circuit 11 for read-out purposes. An electron beam scanningposition timing signal generator 26 is connected to the scanning signalgenerator 22 via the X and Y direction deflection amplifiers 19 and 20in order to develop the timing pulse 13 which is applied to the gatecircuit 11. The gate circuit 11 is connected to the thin-film EL element8 via the amplifier 10.

In synchronization with the synchronization signal derived from thescanning signal generator 22, the sustaining voltage pulse of theamplitude V_(s) is applied from the alternating sustaining voltagesource 7 to the thin-film EL element. At this moment, the elementexhibits the erase brightness B_(e) shown in FIG. 2. A desired patternsignal is developed from the modulator 21 and applied to the scanningsignal generator 22. The electron beam generator 24 generates anelectron beam to be applied to the thin-film EL element 8 disposed atthe display surface of the cathode-ray tube 15.

The electron beam generated from the electron beam generator 24 isfocused by the focus control electro-magnetic coil 16 and directed tothe thin-film EL element 8. The scanning signal generator 22 functionsto control the strength of the electron beam, whereby the brightness ofthe electroluminescence is controlled. The application of the electronbeam is timed in agreement with the application of the alternatingsustaining voltage pulse. The electron beam is applied to the thin-filmEL element 8 through the rear metal electrode 6. The position to whichthe electron beam is applied is controlled through the use of the X-Ydeflection coil 17.

A position where the electron beam is impinged exhibits the brightnessB'_(w) at the point P shown in FIG. 2 and, then, the position ismaintained at the write-in brightness B_(w) by the following sustainingvoltage pulse.

The position to which the electron beam is not applied is maintained atthe erase brightness B_(e). Accordingly, the display pattern is observedthrough the glass substrate 1. The display pattern can be characters,drawings, symbols or continuous patterns. The brightness is easilyvaried by controlling the strength of the electron beam. The entiredisplay surface can be placed in the luminescent condition.

When an erase voltage pulse is applied from the erase signal generator23, the entire display surface of the thin-film EL element is placed inthe erase condition.

When the electron beam is applied to the element at a time when thealternating sustaining voltage signal bears the zero level, the positionto which the electron beam is impinged is placed in the erase condition.

The read-out operation is conducted by applying the electron beam to thethin-film EL element at a time when the alternating sustaining voltagetakes the zero level.

The electron beam scanning position timing signal generator 26 developsthe timing pulse 13, and the inverter circuit 25 develops the timingpulse 12. Accordingly, the gate circuit 11 develops the polarizationrelaxation current signal 14 indicative of the information stored at aposition on the thin-film EL element 8 to which the electron beam isapplied. When the electron beam is applied to a desired position of thethin-film EL element 8 for the read-out purposes, the position is placedinto the erase memory condition.

The write-in operation can be alternatively conducted by applying alight pattern to the thin-film EL element 8 through the glasssubstrate 1. The read-out operation is conducted by applying theelectron beam to the thin-film EL element 8 through the rear metalelectrode 6.

In the foregoing embodiment, the position held in the luminescencememory condition is placed into the erase condition, when the read-outoperation is conducted. In the following embodiment, the luminescencememory condition is maintained even when the read-out operation isconducted.

FIG. 6 shows a basic construction of another embodiment of a drivesystem of the present invention, which employs a thin-film EL element 27having matrix shaped electrodes X₁ -X₃ and Y₁ -Y₃.

The thin-film EL element 27 has a similar construction as that of FIG.1, but has transparent front column electrodes Y₁ through Y₃, and rearmetal row electrodes X₁ through X₃. An alternating pulse source 28 isconnected to the thin-film EL element 27 to apply the alternatingsustaining voltage signal to the thin-film EL element 27. Load impedancemeans such as resistors R₁ through R₃ are connected to the columnelectrodes Y₁ through Y₃, respectively, for read-out purposes. Selectionswitches SW₁ through SW₃ are connected to the row electrodes X₁ throughX₃ in order to scan the sustaining operation and read-out operation. Thenumber of electrodes is not limited to the embodiment of FIG. 6.

FIG. 7 shows the equivalent circuit of the thin-film EL element. Thethin-film EL element can be considered to have a parallel circuit 33including a non-linear resistor 32 and a capacitor 31. The parallelcircuit 33 is connected to capacitors 29 and 30 in a seried fashion. Thecapacitor 31 corresponds to the capacitive component of the thin-filmluminescent layer 4, and the capacitors 29 and 30 correspond to thecapacitive components of the thin-film dielectric layers 3 and 5,respectively. The non-linear resistor 32 can be considered as resistanceagainst the travel of the conduction electrons in the thin-filmluminescent layer 4.

When the thin-film EL element is in the erase condition, the non-linearresistor 32 takes the resistance value above several tens MΩ. Therefore,the thin-film EL element can be considered to be consisting of thecapacitive component. Contrarily, when the thin-film EL element is inthe written-in condition, the non-linear resistor 32 takes theresistance value of ten and several KΩ. Therefore, the electric currentapplied to the parallel circuit 33 will flow through the non-linearresistor 32.

As discussed above, the thin-film EL element can be considered as a typeof capacitor. When an impedance means such as a resistor is connected tothe thin-film EL element in a series fashion, a differentiation circuitcan be formed, which develops a voltage signal across the impedancemeans. A rectangular waveform output signal can be obtained when avoltage signal having a predetermined inclination in the leading edge isapplied to the differentiation cirucit. In case where the thin-film ELelement is in the erase condition, a clear rectangular waveform outputsignal is obtained because the thin-film EL element acts as a capacitor.However, in case where the thin-film EL element is in the written-incondition, the output signal includes the crest portion near the endportion thereof because the thin-film EL element includes the resistancecomponent therein. The information stored in the thin-film EL elementcan be read out by detecting the above-mentioned crest portion.

FIG. 8 shows an embodiment of a read-out voltage pulse generator whichdevelops a voltage pulse having an amplitude identical with that of thesustaining voltage V_(s) and a leading edge of a predeterminedinclination.

A power source voltage of a same level as that of the sustaining voltageV_(s) is applied to a terminal 34. When an input signal as shown in FIG.9(A) is not applied to a terminal 35, a transistor 36 is OFF, atransistor 37 is ON, and a transistor 38 is OFF. Under these conditions,when a signal as shown in FIG. 9(B) is not applied to a terminal 39,transistors 40 and 41 are OFF. An output terminal 42 connected to thethin-film EL element is grounded through a resistor 43. At this moment,a capacitor 44 is maintained so that the terminal conneccted to thecollector electrode of the transistor 36 is positive and the terminalconnected to the collector electrode of the transistor 37 is negative.Another capacitor 45 is charged to the power source voltage level V_(s)through a diode 46 and the resistor 43.

When the input signal as shown in FIG. 9(A) is applied to the terminal35, the transistor 36 is turned ON and, therefore, the collectorelectrode of the transistor 36 and the base electrode of the transistor37 are maintained at the zero level. The transistor 37 is turned OFFand, therefore, the capacitor 44 is charged so that the terminalconnected to the collector electrode of the transistor 37 becomespositive and the terminal connected to the base electrode of thetransistor 37 becomes negative. Accordingly, the transistor 38 begins tobecome conductive.

At this moment, the capacitor 45, which had already been charged to thepower source voltage level V_(s), is connected to the power supplysource through the short circuit including the transistor 38. Therefore,the cathode terminal of the diode 46 receives a voltage of twice thepower supply level and, hence, the diode 46 is biased backward. Thecapacitor 44 is charged by the twice voltage through the resistor 47.The charging period can be controlled by varying the resistance value ofthe resistor 47 and the capacitance value of the capacitor 44. Thecharging time constant functions to determine the inclination angle ofthe leading edge of the output pulse signal to be applied to thethin-film EL element. When the capacitor 44 is charged to the powersupply voltage level V_(s), the base-collector junction of thetransistor 38 is biased forward and, therefore, the output level doesnot exceed the power supply voltage level V_(s).

When the input pulse as shown in FIG. 9(B) is applied to the terminal 39under the condition where the input pulse as shown in FIG. 9(A) takesthe zero level, the transistors 40 and 41 are turned ON, whereby thedischarge period of the thin-film EL element connected to the outputterminal 42 is reduced. FIG. 9(C) shows an output pulse derived from theoutput terminal 42, the output pulse having a leading edge of apredetermined inclination.

FIG. 10 shows an embodiment of a polarization current detection circuitconnected to the thin-film EL current.

A read-out pulse generator 60 is connected to a thin-film EL element 70which is shown by a capacitor. The read-out pulse generator 60 has asame construction as shown in FIG. 8 to develop the read-out pulsehaving a leading edge of a predetermined inclination. An impedance means80 such as a resistor is connected to the thin-film EL element 70 in aseries fashion. The impedance means 80 represents one of the resistorsR₁ through R₃ shows in FIG. 6. A buffer amplifier 90 is connected to theconnection point of the thin-film EL element 70 and the impedance means80.

An output signal of the buffer amplifier 90 is directly applied to oneinput terminal of a differentiation amplifier 103. The output signal ofthe buffer amplifier 90 is also applied to another input temrinal of thedifferentiation amplifier 103 via a switching element 100 such as atransistor and a resistor 101. The connection point of the resistor 101and the input terminal of the differentiation amplifier 103 is groundedthrough a capacitor 102. An output signal of the differentiationamplifier 103 is applied to one input terminal of a comparator 110,which includes the other input terminal connected to a power supplysource 111. The comparator 110 develops a read-out detection signal 112.

When the read-out pulse as shown in FIG. 9(C) is applied from theread-out pulse generator 60 to the thin-film EL element 70, an electriccurrent flows through the thin-film EL element 70, and a voltage signalis developed across the resistor 80. In case where the thin-film ELelement 70 is placed in the erase condition, the detection output signalis a rectangular waveform of a predetermined amplitude as shown in FIG.9(D). In case where the thin-film EL element 70 is placed in thewritten-in condition, the crest portion is observed in the detectionoutput signal as shown in FIG. 9(E) due to the polarization current.

The switching element 100 is closed in synchronization with the leadingedge of the read-out pulse or slightly after the occurrence of theleading edge of the read-out pulse. The capacitor 102 is charged throughthe resistor 101. The switching element 100 is controlled to open beforethe crest portion appears. Accordingly, the input terminal of thedifferentiation amplifier 103 connected to the switching element 100receives a signal as shown in FIG. 9(F).

A period τ during which the switching element 100 is closed is selectedto satisfy the following relationship.

    τ≃(C.sub.o R.sub.o /3)

where:

R_(o) is a resistance value of the resistor 101; and

C_(o) is a capacitance value of the capacitor 102.

At a time when the polarization current flows through the element, theone input terminal of the differentiation amplifier 103 receives thesignal of the level substantially zero. Therefore, the crest portion isamplified by the differentiation amplifier 103. And, the polarizationcurrent component is developed from the comparator 110.

In the embodiment of FIG. 10, the both input terminals of thedifferentiation amplifier 103 receive the signal derived from the sameelement and, therefore, the accurate read out operation is conducted.

Read out operation will be described in detail with reference to anembodiment of FIG. 6, wherein the thin-film EL element has electrodesformed in a 3 x 3 matrix fashion.

Now assume that write-in operation is conducted to picture points (X₁,Y₁), (X₃, Y₁) and (X₂, Y₂) by applying the electron beam under thecondition where the selection switches SW₁ through SW₃ are closed toapply the sustaining voltage pulse between two terminals A and E. Then,the selection switches SW₁ through SW₃ are opened to electricallyseparate the row electrodes X₁ through X₃. Thereafter, the read-outpulse shown in FIG. 9(C) is sequentially applied to the row electrodesX₁ through X₃. An electric current flows through the resistors R₁through R₃ in response to the application of the read-out pulse to therow electrodes X₁ through X₃.

An example of the read out operation will be described with reference toFIGS. 11(A) through 11(F). FIGS. 11(A) through 11(C) show read-outvoltage signals applied to the row electrodes X₁ through X₃,respectively. FIGS. 11(D) through 11(F) show detection output signalsobtained via the resistors R₁ through R₃, respectively.

The picture points (X₁, Y₁), (X₃, Y₁) and (X₂, Y₂) have writteninformation. The resistors R₁ through R₃ receive displacement currents121, 123 and 125 shown in FIGS. 11(D) and 11(E) and shown in FIG. 11(F),and polarization currents 120, 122 and 124 superimposed on thedisplacement currents as shown in FIGS. 11(D) and 11(E). The detectionoutputs corresponding to the respective picture points can be obtainedby detecting the row electrode to which the read-out voltage pulse isapplied and the column electrode connected to each of the resistors R₁through R₃. For example, the output current 120 corresponds to thepicture point (X₁, Y₁). The output current 122 corresponds to thepicture point (X₃, Y₁), and the output current 124 corresponds to thepicture point (X₂, Y₂). The written portion is not damaged by theabove-mentioned read out operation. And, the erroneous write-inoperation will not be conducted to the erase point by the read outoperation. This is because the read-out voltage pulse has the sameamplitude as the sustaining voltage level V_(s).

FIG. 12 shows another embodiment of the drive system of the presentinvention, which embodies the electron beam write-in operation and theelectrical read-out operation.

The thin-film EL element of the construction as shown in FIG. 6 isdisposed at a display surface of a cathode-ray tube 136. That is, theglass substrate 1 (see FIG. 1) defines the front surface of thecathode-ray tube 136.

A focus control electro-magnetic coil 138 and an X-Y deflection coil 137are disposed as is well known in the art. The focus controlelectro-magnetic coil 138 is connected to receive a control signalderived from an electron beam focus control signal generator 170. TheX-Y deflection coil 137 is connected to receive control signals derivedfrom an X-direction deflection amplifier 180 and a Y-directiondeflection amplifier 181. The amplifiers 180 and 181 are connected toreceive signals derived from a scanning signal generator 160, which isconnected to receive a video signal derived form a modulator 150.

The transparent column electrodes 2 and the rear metal row electrodes 6of the thin-film EL element are connected to a gate driver circuit 190which selects the write-in operation and the read-out operation. Thegate drive circuit 190 is connected to a sustaining pulse signalgenerator 200 and a read-out pulse generator 210. An erase signalgenerator 201 is associated with the sustaining pulse signal generator200. The sustaining pulse signal generator 200 and the erase signalgenerator 201 are connected to receive a synchronization signal derivedfrom the scanning signal generator 160. An electron beam generator 139is disposed at the end of the cathode-ray tube 136. The electron beamgenerator 139 is connected to receive a brightness control signalderived from the scanning signal generator 160.

A detection circuit 211 and a differentiation amplifier/comparator 212of the construction as shown in FIG. 10 are connected to the thin-filmEL element. A read-out signal synchornization detection circuit 213 isconnected to the read-out pulse generator 210, which is connected to thescanning signal generator 160, and the differentiationamplifier/comparator 212, thereby developing a read-out signal 214.

When write-in operation is desired to be performed, the gate drivecircuit 190 functions to pass the sustaining pulse signal toward thethin-film EL element. In synchronization with the signal derived fromthe scanning signal generator 160, the sustaining pulse signal generator200 develops the sustaining pulse voltage to be applied to the thin-filmEL element. The thin-film EL element exhibits the erase brightnessB_(e). A desired pattern signal is developed from the modulator 150. Thescanning signal generator 160 controls to impinge the electron beamgenerated from the electron beam generator 139 at a desired position ofthe thin-film EL element which is positioned on the display surface ofthe cathode-ray tube 136. The point to which the electron beam isapplied exhibits the write-in brightness B_(w) as in the case of theembodiment of FIG. 5.

When erase operation is desired to be conducted, the erase pulse isdeveloped from the erase signal generator 201, thereby electricallyerasing the written information. When read-out operation is desired tobe performed, the gate driver circuit 190 functions to pass the read-outpulse toward the thin-film EL element. The read-out pulse generator 210functions to apply the read-out pulse shown in FIG. 9(C) to thethin-film EL element.

The application of the read-out pulse is scanned through the use of theselection switches SW₁ through SW₃ shown in FIG. 6. Then, the read-outsignal 214 is developed from the read-out signal synchronizationdetection circuit 213, the read-out signal 214 indicating the memorycondition of a point to which the read-out pulse is applied.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications are intended to be included within the scope of thefollowing claims.

What is claimed is:
 1. A drive system for erasing information written ona thin-film EL element including a thin-film EL layer sandwiched betweena pair of electrodes, said drive system comprising:means for applying avoltage signal between said pair of electrodes; means for applying anelectron beam to said thin-film EL element through one of said pair ofelectrodes; and control means for applying said electron beam to adesired position on said thin-film EL element at a time when saidvoltage signal is at a level which will not excite the thin-film ELelement thereby erasing information from said EL element.
 2. The drivesystem of claim 1, wherein said electron beam is applied to thethin-film EL element when said voltage signal takes the substantiallyzero level.
 3. The drive system of claim 1, wherein said control meanscomprise:detection means for detecting the electron beam; andsynchronization means for synchronizing the application of the electronbeam with the application of said voltage signal.
 4. The drive system ofclaim 1, 2 or 3, wherein said thin-film EL element comprises:a thin-filmZnS layer doped with manganese; a pair of dielectric layers formed onboth major surface of said thin-film ZnS layer; a front transparentelectrode formed on one of said pair of dielectric layers; and a rearmetal electrode formed on the other dielectric layer, and wherein saidelectron beam is applied to the thin-film EL element through said rearmetal electrode.
 5. The drive system of claim 4, wherein said thin-filmEL element has the hysteresis characteristics, and said voltage signalcomprises an alternating pulse voltage signal.
 6. The drive system ofclaim 4, wherein said front transparent electrode and said rear metalelectrode are formed uniformly on the entire surface of a display regionof said thin-film element.
 7. A drive system for reading out informationwritten on a thin-film EL element including a thin-film EL layersandwiched between a pair of electrodes, said drive systemcomprising:means for applying a voltage signal between said pair ofelectrodes; means for applying an electron beam to said thin-film ELelement through one of said pair of electrodes; control means forapplying said electron beam to a desired position on said thin-film ELelement at a time when said voltage signal level is substantially zero;and detection means for detecting an electric current flowing throughsaid thin-film EL layer when said electron beam is applied to said thinfilm EL element, thereby reading out information written on said ELcurrent.
 8. The drive system of claim 7, wherein said control meanscomprise:deflection means for scanning said thin-film EL element by saidelectron beam; and synchronization means for synchronizing theapplication of the electron beam with the application of said voltagesignal.
 9. The drive system of claim 7, wherein said detection meanscomprise a selection means for selectively detecting a polarizationrelaxation current flowing through said thin-film EL element.
 10. Thedrive system of claim 7, 8 or 9, wherein said thin-film EL elementcomprises:a thin-film ZnS layer doped with manganese; a pair ofdielectric layers formed on both major surface of said thin-film ZnSlayer; a front transparent electrode formed on one of said pair ofdielectric layers; and a rear metal electrode formed on the otherdielectric beam is applied to said thin-film EL element through saidrear metal electrode.
 11. The drive system of claim 10, wherein saidthin-film EL element has the hysteresis characteristics, and saidvoltage signal comprises an alternating sustaining pulse voltage signal.12. The drive system of claim 11, wherein said voltage signal furthercomprises an erase pulse voltage signal for electrically erasinginformation written on said thin-film EL element.
 13. The drive systemof claim 10, wherein said front transparent electrode and said rearelectrode are formed uniformly on the entire surface of a display regionof said thin-film EL element.
 14. The drive system of claim 13, whereinsaid detection means are correlated with said deflection means fordetermining the position from which the polarization relaxation currentis detected.
 15. A drive system for a thin-film EL element including athin-film El layer sandwiched between a pair of dielectric layers and apair of electrodes formed on both dielectric layers, said thin-film ELelement exhibiting the hysteresis memory function for storinginformation, said drive system comprising:means for generating anelectron beam toward said thin-film EL element; means for deflectingsaid electron beam in order to apply said electron beam to a desiredposition on said thin-film EL element; means for applying an alternatingsustaining voltage signal between said pair of electrodes; means forapplying a read-out voltage signal between said pair of electrodes, saidread-out voltage signal having an amplitude substantially identical withthat of the alternating sustaining voltage signal, and said read-outvoltage signal having a leading edge of a predetermined inclination; andmeans for detecting an electric current flowing through said thin-filmEL element at a time when said read-out voltage signal is appliedbetween said pair of electrodes, whereby the stored information isrecovered.
 16. The drive system of claim 15, wherein said pair ofelectrodes comprise:transparent front column electrodes formed on one ofsaid pair of dielectric layers; and rear metal row electrodes formed onthe other dielectric layer, whereby picture points are determined bysaid column and row electrodes.
 17. A drive system for erasinginformation written on a thin-film EL element including a thin-film ELlayer sandwiched between a pair of electrodes, said drive systemcomprising:means for applying an alternating sustaining voltage signalbetween said pair of electrodes; means for applying an electron beam tosaid thin-film EL element through one of said pair of electrodes; andcontrol means for applying said electron beam to a desired position onsaid thin-film EL element at a time when said voltage signal is at alevel which will not excite the thin-film EL element thereby erasinginformation from said EL element.
 18. A drive system for reading-outinformation written on a thin-film EL element including a thin-film ELlayer sandwiched between a pair of electrodes, said drive systemcomprising:means for applying an alternating sustaining voltage signalbetween said pair of electrodes; means for applying an electron beam tosaid thin-film EL element through one of said pair of electrodes;control meansfor applying said electron beam to a desired position onsaid thin-film EL element at a time when said voltage signal level issubstantially zero; and detection means for detecting an electriccurrent flowing through said thin-film EL layer when said electron beamis applied to said thin-film EL element, thereby reading out informationwritten on said EL element.